Wind and solar energy have grabbed headlines with their meteoric rise in recent years, but hydropower still provides far and away more energy than any other renewable source, accounting for 7 percent of total U.S. production. But there are only so many rivers to dam, and America did a pretty good job of that all throughout the 20th century. So it’s not uncommon to hear hydropower described as being “tapped out,” and indeed growth in hydro capacity has essentially flatlined since the early 1990s.

That’s starting to change, says Jeff Leahey, deputy executive director of the industry group National Hydropower Association. “The policy arena has shifted and there’s been more of a focus on clean energy investments and that’s driving people to look at hydro,” he tells CityLab. “Now we’re seeing that growth curve uptick again, when in the last 20 years it’s been pretty flat.”

As world leaders head to Paris for the most important climate negotiations in years, here are a few reasons why the U.S. should keep hydro in mind as a way to trim carbon emissions out of national energy production.

Steady, zero-carbon energy

Hydropower looks attractive to energy planners because it offers what’s called baseload power: steady and predictable generation that serves as the backbone of a diverse energy mix. Rivers keep on flowing day and night, and that keeps turbines spinning (barring catastrophic droughts, which are becoming more of an issue as climate change kicks up the heat). This steadiness makes hydropower an important counterpoint to the variability of wind and solar energy; their production goes up and down based on the weather, which makes it hard to depend heavily on them for the bulk of power generation.

Hydro is our best form of energy storage

As wind and solar continue their upward growth, the need for large-scale energy storage becomes ever more acute. That’s because we need to be able to store surplus energy from periods of intense wind and sunlight for use when the breeze dies and the sun goes down.

Engineers have developed a whole range of potential technologies for this task (Tesla’s Powerwall, for one, provoked an avalanche of reservations when it became available). But the only technology currently storing energy on a massive scale is as old as Archimedes: pumped water. The U.S. has about 22 gigawatts of pumped storage hydropower, which makes up a staggering 97 percent of utility-scale electricity storage, according to a Department of Energy report from 2014.

The way this works is you have two bodies of water, one higher than the other, with a dam in between them. When the grid is producing more power than what’s being consumed, a pumped storage facility can use that electricity to pump water from the lower basin to the upper one. When the facility needs to recover that energy, it releases the water, which falls to the lower basin, spinning a turbine and creating electricity.

All of this is to say that as variable sources make up a greater share of the nation’s power supply, the gap between high production and low production will increase, and energy suppliers will need more storage to bridge that gap. Pumped hydropower facilities have proven their ability to do just that.

There’s room to expand without building new dams

Constructing new dams is highly difficult, both for the capital costs of the actual construction and because they impose massive changes on a river’s ecosystem. But the vast majority of existing dams in the U.S.—more than 90 percent, or 80,000 dams—don’t produce electricity. They just hold back water, which can be useful for things like flood control, irrigation, and navigation.

“The dam has already been built, the impacts have already been felt,” Leahey says. “Those dams are going to be there for the long haul, but right now they’re just not generating any power. … We think that’s a prime opportunity for near-term growth in the hydropower industry.”

A 2012 Department of Energy report identified a total of 12 gigawatts of new hydropower to be built by retrofitting non-powered dams. That’s 15 percent of current hydropower capacity, or about half of all U.S. solar. And that capacity is more likely to be accepted by groups dedicated to protecting river ecosystems, which new dams do much to disrupt.

Kate Miller, the director of government affairs for one such group, Trout Unlimited, stresses that hydropower dams are uniquely site-specific in their benefits and impacts. But in general, she says, “retrofitting and using existing infrastructure is definitely the best way to go about finding new hydropower energy without building new dams or creating additional disruptions and diversions to waterways.”

All renewable energy projects call for a trade-off between energy benefits and ecological disruption, but in the case of pumped storage, that's a trade the increasingly clean U.S. energy sector should be willing to make.